The present invention relates to the improvements of a variable compression ratio mechanism for a reciprocating internal combustion engine.
In order to vary a compression ratio between the volume existing within the engine cylinder with the piston at bottom dead center (BDC) and the volume in the cylinder with the piston at top dead center (TDC) depending upon engine operating conditions such as engine speed and load, in recent years, there have been proposed and developed multiple-link type reciprocating piston engines. One such multiple-link type variable compression ratio mechanism has been disclosed in pages 706-711 of the issue for 1997 of the paper xe2x80x9cMTZ Motortechnische Zeitschrift 58, No. 11xe2x80x9d. The multiple-link type variable compression ratio mechanism disclosed in the paper xe2x80x9cMTZ Motortechnische Zeitschrift 58, No. 11xe2x80x9d is comprised of an upper link mechanically linked at one end to a piston pin, a lower link mechanically linked to both the upper link and a crankpin of an engine crankshaft, a control shaft arranged essentially parallel to the axis of the crankshaft and having an eccentric cam whose axis is eccentric to the axis of the control shaft, and a control link rockably or oscillatingly linked at one end onto the eccentric cam of the control shaft and linked at the other end to the lower end of the upper link. In order to vary the attitude of each of the upper and lower links, the other end of the control link may be linked to the lower link, instead of linking the control link to the upper link. By way of rotary motion of the control shaft, the center of oscillating motion of the control link varies via the eccentric cam, and thus the distance between the piston pin and the crankpin also varies. In this manner, a compression ratio can be varied. In the reciprocating engine with such a multiple-link type variable compression ratio mechanism, the compression ratio is set at a relatively low value at high-load operation to avoid undesired engine knocking from occurring. Conversely, at part-load operation, the compression ratio is set at a relatively high value to enhance the combustion efficiency.
In order to produce the rotary motion of the control shaft, a control-shaft actuator is used. The control-shaft actuator is often comprised of a control screw portion and a control nut portion engaged with each other. Suppose that an external screw-threaded portion, serving as the control screw portion, is provided on a reciprocating block slider of the actuator, whereas an internal screw-threaded portion, serving as the control nut portion, is provided in a cylindrical member of the actuator. When the cylindrical member is driven in its one rotational direction by means of a power source such as an electric motor or a hydraulic pump, one axial sliding movement of the reciprocating block slider occurs by way of the control screw portion and the control nut portion. Conversely when the cylindrical member is driven in the opposite rotational direction, the opposite axial sliding movement of the reciprocating block slider occurs by way of the control screw portion and the control nut portion. During operation of the reciprocating engine with the multiple-link type variable compression ratio mechanism, owing to a piston combustion load (compression pressure) or inertial load of each of the links, a load acts upon the eccentric cam of the control shaft through the piston pin, the upper link and the control link. That is, owing to the piston combustion load, torque acts to rotate the control shaft in a rotational direction and thus a reciprocating load acts to move the reciprocating block slider in its axial directions. The torque acting on the control shaft will be hereinafter referred to as a xe2x80x9ccontrol-shaft torquexe2x80x9d. The reciprocating load mostly acts in a principal direction, that is, in a direction of the force acting on the reciprocating block slider owing to the piston combustion load. However, at a timing wherein the piston combustion load is less and the inertial load is great, the reciprocating load tends to act in a direction opposite to the principal direction. If the direction of reciprocating load acting on the reciprocating block slider is reversed, there is an increased tendency for the reciprocating block slider to oscillate within a backlash (defined between the internal and external screw-threaded portions) axially relative to the cylindrical member (rotary member) of the actuator. Owing to reversal of the direction of reciprocating load acting on the reciprocating block slider, there is a possibility of collision between the face of tooth of the inner screw-threaded portion and the face of tooth of the external screw-threaded portion, that is, undesired hammering noise and vibration.
Accordingly, it is an object of the invention to provide a variable compression ratio mechanism for a reciprocating internal combustion engine, which avoids or suppresses hammering noise and vibration to occur owing to a backlash defined between internal and external screw-threaded portions being in meshed-engagement with each other and constructing part of a control-shaft actuator.
In order to accomplish the aforementioned and other objects of the present invention, a variable compression ratio mechanism for a reciprocating internal combustion engine including a piston moveable through a stroke in the engine and having a piston pin and a crankshaft changing reciprocating motion of the piston into rotating motion and having a crankpin, the variable compression ratio mechanism comprises a plurality of links mechanically linking the piston pin to the crankpin, a control shaft to which an eccentric cam is attached so that a center of the eccentric cam is eccentric to a center of the control shaft, a control link connected at one end to one of the plurality of links and connected at the other end to the eccentric cam, and an actuator that drives the control shaft within a predetermined controlled angular range and holds the control shaft at a desired angular position so that a compression ratio of the engine continuously reduces by driving the control shaft in a first rotational direction and so that the compression ratio continuously increases by driving the control shaft in a second rotational direction opposite to the first rotational direction, the actuator comprising a reciprocating block slider linked at a first end portion to the control shaft, a rotary member being in meshed-engagement with the second end portion of the slider by a meshing pair of screw-threaded portions, so that rotary motion of the rotary member is converted into axial sliding motion of the slider to drive the control shaft in one of the first and second rotational directions, and a hydraulic pressure chamber facing an axial end face of the second end portion of the slider, so that working-fluid pressure in the hydraulic pressure chamber forces the slider in the same axial direction as a direction of action of a reciprocating load acting on the slider during down stroke of the piston, the reciprocating load acting on the slider in axial directions of the slider during up and down strokes of the piston.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.